To weigh masses with high accuracy, the state-of-the-art solution is to use balances that are based on the principle of magnetic force compensation. In balances of this kind, the force acting on the weighing pan is transmitted by guide members and through flexure pivots to a lever system. The last lever of the lever system is held in a constant position by means of a force coil immersed in a permanent magnet. The required amount of current flowing through the force coil is measured and used to determine the weighing result.
Conventional magnetic force compensation balances isolate all moments acting on the weighing pan, so that only the force in the direction of gravity is being measured. This concept has the disadvantage that additional forces could arise in the direction of gravity and falsify the weighing signal. Consequently, the balance should be set up in such a way that the line of action of the weighing cell, i.e. the axis along which the force under measurement is to be applied and along which the weighing cell has its highest sensitivity is aligned as closely as possible to the direction of gravity. This is accomplished by adjusting the leveled position of the balance.
In addition, further errors can occur if the center of gravity of the weighing object is not located on the line of action of the weighing cell. The associated eccentric loading force is not transmitted to the weighing cell and cannot be taken into consideration for the measurement. Eccentric loading forces can only be estimated and corrected with additional sensors.
It is further disadvantageous that forces can occur in the various pivot points and flexure joints, whereby the accuracy of the balance can be compromised. Furthermore, levers and flexure pivots are particularly sensitive to heat and therefore often become sources of errors for the weighing result.
Among the prior art, a balance with an electromagnetic force compensation system is disclosed in U.S. Pat. No. 5,485,784 wherein the weight force is transmitted directly from the weighing pan to the magnetic force compensation system without mechanical contact. Thus, there is no loss of force of the kind occurring in the pivot points and transmission levers of traditional magnetic force compensation balances. The gravity force is compensated by a system with two force coils. In addition, passive magnets are arranged vertically, serving to stabilize the force compensation system in the direction of gravity. Thus, the passive magnets ensure the correct leveled position of the balance. In addition, the passive magnets compensate for the moments that are caused by the eccentrically placed load. In this set-up, the compensation depends only on the magnet force and on the arrangement of the passive magnets relative to each other. It is not matched to the eccentric loading force. The extent to which the eccentric loading force affects the weighing result can therefore not be estimated. Consequently, this system is too inaccurate for balances of the highest resolution of the weighing result.
Disclosed in JP2005127858A is a weighing device with at least six magnet units and guide elements holding a load chamber in a floating position free of contact. Each of the magnet units includes two coils, two electromagnets and a permanent magnet. Arranged opposite each magnet unit is a guide element of ferromagnetic material which is fastened to the load chamber. The magnet units are arranged relative to the guide elements in such a way as to prevent any translatory or rotary movement of the load chamber. The distance between each guide element and the corresponding magnet unit is measured with one position sensor per magnet unit.
After the weighing object has been placed in the load chamber, the current is regulated in order to stabilize the distance between magnet units and guide elements. When the position is stabilized, the current is regulated gradually towards zero. The attractive forces of the magnet units change accordingly, and as a consequence the distance between the magnet units and the guide elements, and thus the position of the load chamber, is changed. When the current has reached zero, the distances between the magnet units and the respective guide elements are measured. The weight of the weighing object is calculated from the distances between the magnet units and the respective guide elements and from the permeability of the magnet. A Hall element is used to optimize the regulation of the current and thus of the distance between the magnet units and guide elements.
The weighing device disclosed in JP2005127858A has the advantage that the current in the coils is set to zero. Thus, no additional heat is generated in the coils. The influence on the properties of the magnets is therefore limited.
However, the determination of the weight based on the distance has considerable disadvantages. The nonlinearity of the functional relationship between magnetic field and distance makes it on the one hand more difficult to regulate the distance and on the other hand leads to an inaccuracy in the determination of the weight.
It is a further drawback of this balance to have an enclosed load chamber which is carried by magnets on all sides. The enclosed load chamber is heavier than a weighing pan. Due to its so-called dead load, the load chamber limits the maximum volume of the weighing object. Furthermore, the magnets are attached to the load chamber. Consequently, the weight to be carried besides the weighing object is heavier, and the accuracy of the balance is reduced.
The load chamber has the further disadvantage that a balance of this kind is very voluminous, independent of the volume of the load, and is therefore strongly influenced by air buoyancy. In addition, due to the large volume this solution is not suitable for analytical balances.
It is therefore the object of the invention to provide a weighing method and a weighing device for carrying out the method wherein no internal reactive forces are present, which allows the weight to be measured with the highest possible accuracy, independent of the volume of the weighing object, and wherein the weighing device is as insensitive to vibrations as possible. A weighing method is intended to measure, and to compensate for, the position of the center of gravity of the weighing object relative to the weighing pan as well as the out-of-level condition of the weighing device.